Remember that just because you see "bubbles" maybe coming out from your diffusor etc and floating to the top doesn't mean that the water isn't saturated with CO2. At the molecular level CO2 bonds are breaking and reforming with H2O, all invisible to the naked eye.

Since the fish tank is essentially a container with an open top the partial pressure law will have the CO2 exiting the top of the tank to mix with the atmosphere "degassing" etc. So towards your question, I would say that with a constant flow of CO2 into a diffusor all the plants in the tank are getting almost equal CO2. BUT, I would be curious myself to test as an experiment in an aquarium the levels of carbonic acid at the surface and at the bottom of an aquarium say 20" deep, I'd like to see some figures.